Fuel cells can be used as a power source in many applications. For example, fuel cells have been proposed for use in automobiles as a replacement for internal combustion engines. In proton exchange membrane (PEM) type fuel cells, a reactant such as hydrogen is supplied as a fuel to an anode of the fuel cell, and a reactant such as oxygen or air is supplied as an oxidant to the cathode of the fuel cell. The PEM fuel cell includes a membrane electrode assembly (MEA) having a proton transmissive, non-electrically conductive, proton exchange membrane. The proton exchange membrane has an anode catalyst on one face and a cathode catalyst on the opposite face. The MEA is often disposed between porous diffusion media such as carbon fiber paper, which also facilitate a delivery of the reactants.
In a fuel cell stack, a plurality of fuel cells is aligned in electrical series, while being separated by gas impermeable, electrically conductive bipolar plates. Each MEA is typically sandwiched between a pair of the electrically conductive bipolar plates. The bipolar plates have an array of grooves or channels that form flow fields for distributing the reactants over the surfaces of the respective anodes and cathodes. Tunnels are also internally formed in the bipolar plate and distribute appropriate coolant throughout the fuel cell stack, in order to maintain a desired temperature.
It is known to nest the channels in active regions of the fuel cell stacks to reduce the amount of coolant mass, and thereby minimize the overall thermal mass, of the fuel cell stack. For example, fuel cell stacks with nested bipolar plates are described in commonly-owned U.S. Pat. No. 6,974,648 to Goebel and U.S. Pat. No. 7,291,414 to Goebel et al., the entire disclosures of which are hereby incorporated herein by reference. Typically, nested channels do not allow adequate space for diffusion media in the feed or cross-flow regions.
Channels are generally not nested in feed regions of the fuel cell stacks so that reactants may be directed to and from headers of the bipolar plate. It is also known to remove the diffusion media from the feed regions in order to provide further space for fluid flow through the non-nested channels. However, fuel cells with feed regions without diffusion media are unsupported. The unsupported feed regions have created challenges for the design of softgood architecture, as sub-gasket materials which are thick enough to span weld glands may cause lamination and durability issues for the proton exchange membrane at the edges of the sub-gasket.
The use of diffusion media and shims for seal support in the tunnel regions of the fuel cell stacks is described in U.S. Pat. Appl. Publ. No 2007/275288 to Goebel et al., the entire disclosure of which is hereby incorporated herein by reference. A further known alternative is to have half height channels in the feed regions while retaining diffusion media. However, these designs may result in an undesirable pressure drop in the feed region of the fuel cell stack, where only one channel depth of space is known to exist due to the nesting of channels. With diffusion media in the feed regions, the cross-flow channels must share the available depth of nominally one channel depth, or nominally half the normal channel depth for feed channels in each bipolar plate. The active area is unchanged, however, and consequently has the same pressure drop. Therefore, the increase in pressure drop is within the feed region. The pressure drop scales with hydraulic diameter to the 4th power, so the half height channels have a significant impact on pressure drop in the feed region of the fuel cell stack.
There is a continuing need for a system providing membrane support within the feed regions of a nested bipolar plate of a fuel cell stack while minimizing a thermal mass thereof. Desirably, the system also enables a seal on softgoods and facilitates an elimination of a subgasket from the fuel cell stack.